LU-4131 mdd: Changelog not logging file truncate.

LU-4131 mdd: Changelog not logging file truncate.

Use of truncate(2) does not produce any change log entries,
and the action is invisible. This patch introduces CL_TRUNC
event and adds the corresponding changelog record when the
size has changed.

Signed-off-by: Rahul Deshmukh <rahul_deshmukh@xyratex.com>
Change-Id: I6621e726a82a8105a959b77b4f505f42990f5d64
Reviewed-on: http://review.whamcloud.com/9723
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>